I didn't make whole language service to not start since I wasn't sure end result of
such big change. that probably should be done by IDE team.

Address PR feedback and move the project filtering up the callstack into StartPushingToWorkspaceAndNotifyOfOpenDocuments_Foreground.

The dependency projects were added as a way to help unify our project.json references.  Instead of every project referencing the NuGet package, we had a dependency project which referenced it and everyone referenced that dependency project.  Due to the transitive nature of NuGet it would be roughly equivalent.

This system had a number of downsides:

1. There was no enforcement.  Even though dependency projects existed, there was no mechanism to force developers to use them.
2. Getting the granularity correct was tricky.
3. The projects themselves have special props / targets to get them to produce no output.  This can trip up MSBuild up to date checks.

Since then though we've begun using RepoUtil to verify our NuGet reference correctness.  This removed the need for these projects and hence I'm removing them from our build.

Check that the project being enabled for pushing events to workspace hostw is still being tracked by the project tracker

[StartPushingToWorkspaceAndNotifyOfOpenDocuments](http://source.roslyn.io/#Microsoft.VisualStudio.LanguageServices/Implementation/ProjectSystem/VisualStudioProjectTracker.cs,187e105ec91d077b,references) might be invoked from a background thread, and hence scheduled to be executed later on the foreground task scheduler. By the time it gets scheduled, we might have removed the project from the tracker on the UI thread. We now guard against this condition by filtering out the removed projects from the push list.
Fixes VSO #[271781](https://devdiv.visualstudio.com/DevDiv/_workitems?id=271781)

Rename intenral code to 'FindUsages' so it better reflects that it can present results for more than just FindReferences.

Fixes #15217

Also, change the column header from "Style" to "Required Style"

Our references were previously a mix of preview 4 and some build between preview 4 and 5.  That was causing build output double writes.

Fixes part 2 of #9674 for both the general code style page and the
naming style option page.

This also removes the superfluous "EnforcementLevel" class in the
NamingPreferences namespace that was basically a duplication of
"NotificationOption"/"NotificationOptionViewModel" in the general
CodeStyle namespace.

Fixes #12669. Addresses one part of #9674

Fixes both the general code styles page and the naming styles page